Safety of operation of nuclear fission power reactors is one of the utmost concerns for the Nuclear Regulatory Commission, the operating utility, and the public. This includes the reliability of the reactor containment should some malfunction occur. This also includes the reliability and accuracy of the reactor controls relating to the actual operating conditions in the reactor. This is needed in order to provide prompt and appropriate corrective measures should some malfunction occur in order to minimize reactor damage and to maximize public safety.
The conventional water-cooled nuclear fission reactor has a reactor core and fuel and control elements are arranged in a matrix fashion in the core. By moving the control elements axially within the core the fission reaction of the fuel elements is regulated, as is the generation of heat. Two common water-cooled reactors are the pressurized-water and the boiling-water types. This invention illustrates application to the pressurized-type.
The core of the pressurized-water reactor is held in a liquid tight vessel, and cooling water is initially directed via a "downcomer" region adjacent the vessel wall to a lower plenum underlying the reactor core, is directed then upwardly through the core and over the fuel elements to an upper plenum overlying the core, and is then directed to exterior heat exchanger means for generating steam.
Different off-normal conditions could exist where: (1) water could be in the downcomer region but not within the core; or (2) vice versa; or (3) where the water could be at some intermediate level in the vessel, such as in the lower plenum, in the core, in the upper plenum, or in the head. In addition, the density of the water within the vessel can possibly vary if steam mixes with the water, which can be of great significance depending on the type, design, and normality of these cooling conditions. Thus, not only must the presence and/or lack of water in the reactor vessel broadly be confirmed, but such information for all or any of these specific regions would be useful to minimize uncertainties and provide signal correlation as to the operation of the reactor cooling system. Expanded efforts have thus since been directed to provide better and redundant detection of the reactor cooling system including specifically the level of the coolant water therein.
Several different types of internal or intrusive monitors have been proposed, using detectors for measuring such properties as (1) water pressure or (2) the presence of neutrons emitting from nuclear reactions. With the internal monitors, the detectors are within the reactor vessel while the readout devices are located outside of the reactor vessel. Conduits must therefore be used between the detectors and readout devices, and these conduits must pass through the reactor vessel itself and possibly other pressure boundary structures; this forms one major drawback to internal monitors. Another major drawback to internal monitors is the survival of the detector itself, located within the confines of the reactor vessel, through the cause or effect of the accident that might be responsible for the change of water level in the first place. Furthermore, pressure monitors prove insensitive in distinguishing between water and water-steam mixtures; and neutron detection has heretofore been of limited precision in detecting the water level inside the vessel.
External or nonintrusive monitors are also available, where the detector and readout device are located outside of the reactor vessel, so that the drawbacks of internally located detectors are eliminated. One such external control is known as a source range or low intensity neutron detector. The detector is designed to respond to low start-up power levels, perhaps 1-10% of full output power. Although the output signals change with changing water levels in the reactor vessel, the resolution is too poor to advise with sufficient reliability and accuracy of the water level in the reactor.
Intrusive and nonintrusive instrumentation each has generic virtues and each must cope with problems arising from differences in flow, turbulence, phase changes, hydraulic constraints, local heat or neutronic conditions, and effects due to fuel management. Each must address the need for data in the reactor head and downcomer regions, and elsewhere in the core. The main generic advantages of nonintrusive monitoring systems are lack of penetration into the pressure boundary and inherent nondestructibility.
My Pat. No. 4,092,542 issued May 30, 1978 and titled "High-Resolution Radiography by Means of a Hodoscope" illustrates a nonintrusive scanning system that is related to the invention to be disclosed herein.